The present invention relates to a switching circuit advantageously applicable to a record/playback mode switching circuit of a video tape recorder.
With a modern broad-band video tape recorder (simply referred to as a VTR hereinafter), it is possible to record and reproduce a multiplex signal directly on and from a magnetic tape. This allows characters information, graphic information, VITC (Vertical Internal Time Code) signal or similar multiplex signal carrying desired information to be overwritten in a magnetic tape in which a video signal has been recorded. A multiplex signal and a video signal will be successfully combined if a combining system consisting of a personal computer, word processor software and some hardware, and system software for controlling the combining system is developed and used. Specifically, a multiplex signal may be combined with a video signal by preparing two VTRs and the combining system stated above, loading one VTR with an original tape in which a video signal has been recorded, playing back the original tape, feeding the resultant video signal to the combining system, adding the multiplex signal to the video signal in the combining system, and recording the resultant combined signal in another tape by the other VTR. Such an approach is undesirable, however, because the video signal is dubbed in another tape resulting in the picture quality being critically degraded.
In the light of the above, there has been proposed a method which overwrites a character multiplex signal or a VITC signal, for example, in a recorded magnetic tape locally and forcibly at an adequate position within a vertical blanking according to standards. Although such a scheme prevents a video signal from being dubbed in and, therefore, eliminates the degradation of picture quality, it is not practicable unless at least the record/playback mode is switched over at high speed. More specifically, the playback and record mode have to be set up alternately within a switching time of less than 100 nanoseconds only during the period of several lines in a vertical blanking in which the multiplex signal should exist. The record/playback mode switchover is usually implemented by a switching circuit which connects one of a record and a playback amplifier to ground and the other to a video head. While various kinds of switching circuit have been proposed for the above application such as a circuit using a relay and a circuit using transistors, they are not capable achieving a satisfactorily high switching speed.